Guest Checkout and email OTP refactor

• Removed the need for users to create an account before browsing or ordering.

• Kept the storefront experience frictionless: users can still browse products and add items to the cart without logging in.

• Added checkout.html

• Added a checkout flow that collects:
  full name, phone number, email, 6-digit OTP code

• Added a “Send Code” button in the checkout form to trigger email verification before order placement.

• Implemented OTP storage with a temporary in-memory map and expiration logic.

• Added backend endpoints for:sending OTP emailsverifying OTP codessubmitting guest checkout orders after successful validationUpdated the order flow to store guest customer data directly in the database instead of requiring a registered user session.

• Kept the storefront experience frictionless: users can still browse products and add items to the cart without logging in.

• REMOVE auth.html